1. Field of the Invention
This invention relates to a semiconductor element, particularly usable for a field-effect transistor (FET), a high electron mobility transistor (HEMT), a heterojunction bipolar transistor (HBT) or the like.
2. Related Art Statement
With the recent development of cellular phone technique and optical communication technique, low electric power consumable and high output electron devices having high frequency performance are remarkably desired. Also, with the recent development of energy saving, large electric power electron devices having large voltage-resistance are remarkably desired. As such an electron device, conventionally, a Si device and a GaAs device have been employed. However, since these device can not exhibit adequate performances as such an electron device, another suitable electron device is keenly desired as the electron device.
In this point of view, a HEMT and a psudemorphic HEMT which are made of GaAs-based semiconductors have been developed and practically used. Moreover, a high performance electron device such as a HEMT and a HBT made of InP-based semiconductor have been actively researched and developed.
However, such a high performance electron device is composed of plural semiconductor layers epitaxially grown on a given substrate, and has a much complicated structure. Moreover, micro processing technique is required in fabricating the electron device, so that the manufacturing cost of the electron device rises. In addition, the InP-based semiconductor is very expensive, so that alternatives are desired.
In this point of view, recently, much attention is paid to a new electron device made of a GaN-based semiconductor. Since the bandgap of the GaN semiconductor is 3.39 eve, the GaN semiconductor can have a dielectric breakdown voltage tenfold as large as that of GaAs semiconductor and Si semiconductor. Moreover, since the GaN semiconductor can have a large electron saturated drift velocity, it can have a larger performance index as an electron device than the GaAs semiconductor and the Si semiconductor. Therefore, the GaN semiconductor is prospected as a fundamental semiconductor for a high temperature device, a high output device and a high frequency device in engine controlling technique, electric power converting technique and mobile communication technique.
Even though such an electron device is made of the GaN-based semiconductor, however, the designed performance index of the electron device can not be realized.
It is an object of the present invention to provide a semiconductor element usable as such an electron device which can exhibit a large performance index designed and an epitaxial substrate preferably usable for the semiconductor element.
In order to achieve the above object, this invention relates to a semiconductor element including a conductive SiC base having a resistivity of less than 1xc3x97105 xcexa9cm, an underlayer made of a semiconductor nitride including at least Al element which is formed on the SiC base, and a semiconductor nitride layer group made including at least one of Al element, Ga element and In element.
This invention also relates to an epitaxial substrate including a conductive SiC base having a resistivity of less than 1xc3x97105 xcexa9cm, and an underlayer made of a semiconductor nitride including at least Al element which is formed on the SiC base.
The inventors had intensely researched the cause that an electron device made of a GaN-based semiconductor can not exhibit the designed performance index and thus, can not realize a sufficient output performance and a high frequency performance. As a result, they found out the following fact of the matter.
Although a conventional electron device is made of a GaN-based semiconductor layer group which is formed on a given substrate, as of now, a sapphire single crystal substrate is employed because it is not expensive. However, the thermal conductivity of the sapphire single crystal substrate is much lower than that of any other single crystal substrate. Therefore, a heat generated during the use of the electron device can not be radiated sufficiently and stored in the electron device. As a result, the physical properties of the semiconductor layer group is shifted from the originally designed physical properties, and thus, the electron device can not exhibit the desired performance index.
In this point of view, the inventors have made an attempt to practically use a SiC substrate as a new substrate material instead of the sapphire single crystal substrate. Since the SiC substrate has a large thermal conductivity, the performance index of an electron device constructed of the SiC substrate is not degraded due to the thermal storage.
However, the kind and the amount of impurity contained in the SiC substrate fluctuates due to the fabricating method, and thus, the conductive property of the SiC substrate fluctuates largely. Normally, an insulative and low conductive SiC substrate is expensive because the fabricating process becomes complicated. Therefore, an electron device constructed of such an insulative SiC substrate is also expensive, and is not suitable for mass-production.
On the other hand, since a conductive SiC substrate can be easily fabricated, it is not expensive and suitable for mass-production. Therefore, the fabricating cost of electron device can be lowered if the electron device is constructed of the conductive SiC substrate. However, the high frequency performance of the electron device may be deteriorated due to the conductivity of the SiC substrate, and thus, the conductive SiC substrate can not be practically employed for the electron device, as of now.
In this point of view, the inventors had made an attempt to practically use the conductive SiC substrate not expensive. As a result, if an underlayer is made of an Al-including semiconductor nitride on the conductive SiC substrate and then, a given semiconductor layer group functioning as a semiconductor element is formed on the underlayer, the deterioration of the thus obtained electron device can be repressed due to the large insulation of the underlayer.
In the present invention, the above-mentioned semiconductor layer group is required to be a semiconductor nitride layer group including at least one of Al element, Ga element and In element. The requirement is originated from the large performance index and the epitaxial growth of the semiconductor layer group on the underlayer.
The present invention can be applied for various semiconductor device, particularly usable for an electron device such as a FET, a HEMT and a HBT.